1. Field of the Invention
The present invention relates to a light guide plate utilizing diffraction gratings for controlling of light emissions, and a backlight module for a liquid crystal display using such a light guide plate.
2. Description of Prior Art
A typical liquid crystal display requires a backlight module in order to be able to provide uniform illumination. The performance of the backlight module greatly depends on a light guide plate employed therein. Means for enhancing the uniformity of light that is output from a light guide plate can be classified into two categories. The first category uses geometrical optic means, such as prisms or micro dots. The second category uses wave optic means, such as diffraction gratings. Light guide plates with multifarious configurations of micro dots and prisms have been developed, and some of these light guide plates can generate quite uniform light beams. However, the uniformity provided by dots is relatively low compared with light guide plates having gratings. This is because the gratings of the latter kind of light guide plate can be precisely configured to correspond to the wavelength band of visible light beams, thereby accurately controlling the uniformity of transmission of the light beams. Nevertheless, there are two main problems associated with gratings. Firstly, a grating is subject to becoming worn over time. Secondly, a grating generates spectral phenomenon.
Referring to FIG. 5, U.S. Pat. No. 5,703,667, issued on Dec. 30, 1997, discloses a backlight module. The backlight module 1 comprises a light guide plate 2 having a light incidence surface 21, a bottom surface 22 and a light emitting surface 23, a fluorescent tube 4 disposed adjacent the light incidence surface 21, a reflection plate 5 disposed under the bottom surface 22, and a diffusing plate 6 and a prism plate 7 disposed on the light emitting surface 23 in that order from bottom to top.
A plurality of reflective diffraction grating units 3 is provided on the bottom surface 22. All the diffraction grating units 3 are parallel with the fluorescent tube 4. Each diffraction grating unit 3 comprises a grating part parallel with the fluorescent tube 4, and a non-grating part. Because all the grating parts of the diffraction grating units 3 are aligned in parallel as described, the diffraction grating units 3 provide strong diffraction of light beams orthogonally emitting from the fluorescent tube 4.
The ratio of a grating part width to a non-grating part width in the diffraction grating units 3 becomes progressively greater with increasing distance away from the light incidence surface 21. Therefore, light beams that are available in large quantities at places nearer to the light incidence surface 21 undergo weaker diffraction, and light beams that are available only in small quantities at places more remote from the light incidence surface 21 undergo stronger diffraction. As a result, the light emitting surface 23 provides uniform outgoing light beams.
However, in numerous actual applications, the linear fluorescent tube 4 cannot accurately emit light rays in a same direction, or point light sources are used instead of the linear fluorescent tube 4. In such cases, the light beams arriving at the bottom surface 22 do not have a same direction. If the light beams are mainly incident on a diffraction grating unit 3 at an angle that is other than orthogonal, the diffraction effect of the diffraction grating unit 3 is weak. Furthermore, light beams arriving at the non-grating parts of the diffraction grating units 3 are wasted. As a result, the light guide plate 2 has limited uniformity of outgoing light beams and limited efficiency of utilization of light.
It is desired to provide a backlight module having a light guide plate which overcomes the above-described problems.